This invention relates to devices in which a flexible member such as a line is partially wrapped for rotatable support, such as in a pulley or sheave, otherwise known as a bearing block.
A conventional sheave or pulley comprises a rigid inner circular race, an outer rigid race having an outwardly facing groove to receive a line, and some type of bearing means between the inner and outer race, such as low friction sleeve or ball, roller or other type of bearings. The outer race is frictionally engaged by a line and rotates around the inner race. Pulleys and sheaves are employed to change the direction and angle of pull of a movable line under tension. The load is transferred from the outer race, through the bearings into the inner race, and the inner race is attached to a support.
High capacity sheaves used on sailing vessels will typically use roller or other high strength bearings to carry the axial loads, with the bearings being composed of a metal or high strength self-lubricating polymer.
In connection with conventional blocks or sheaves, it has been found that the bearings incorporated to withstand the axial load are not loaded equally. For example, assume a conventional sheave using roller bearings and having a line wrap under tension around the outer race of 180 .degree. between, for example, 90.degree. and 270.degree.. The bearing or bearings at the 180.degree. position carry the maximum load, with the loads decreasing on bearings to either side and decreasing to zero at the 90.degree. and 270.degree. locations. The remaining bearings, being located outside the zone of engagement with the line, are not under compression and do not carry any load.
From the above considerations, it will be appreciated that the most highly stressed or loaded bearings in a conventional sheave are located midway between the points where the line enters and exits from the sheave. As a consequence, the strength of the most highly stressed bearings determine the ultimate or maximum working strength of the sheave. In order to increase load capacity, stronger or heavier materials must be used, or additional bearings must be employed. Typically, this would require that the overall size or diameter of the sheave be increased.
Another concern of sheaves of this nature is the potential for the most highly stressed bearings to permanently distort when subjected to high static loads over a long period of time. For example, many sheaves and blocks used on sailing vessels use bearings made of self-lubricating polymeric materials. In many cases, the lines are sheeted or secured under high tension for long periods of time. This may cause permanent deformation of one or more bearings in the critical zone, which greatly impairs free rotation of the outer race.
Finally, it would be desirable in many cases to provide a sheave or pulley or its equivalent that is lighter in weight and/or smaller in size than conventional devices, but still having the same strength or load capacity. This is especially true in racing sailboats, in which windage and weight are important considerations.